The present invention relates generally to flame arrestors, and more particularly to a flame arrestor arrangement for the inlet tube or other intake of an aircraft fuel pump.
Aircraft fuel systems conventionally employ multiple fuel tanks which may be mounted onboard in the wing or fuselage. The tanks typically are connected by transfer tubes, and by venting ducts which maintain atmospheric pressure in the tanks under normal flow conditions. In many fuel systems, transfer pumps are mounted on wing spars outside the wings to move fuel from one tank to another in order to xe2x80x9ctrimxe2x80x9d the aircraft. Smaller, xe2x80x9cscavengexe2x80x9d pumps also may be provided within the tanks to empty residual fuel after the remainder of the fuel has been drawn down to the level of the inlets of the principal transfer pumps. Pumps also are used to transfer fuel from remote tanks to the engine.
Accordingly, a number of fuel pumps, which may be mounted externally of the tank or, alternatively, internally mounted and submersed within the tank, typically are carried as on-board equipment in any given aircraft. In basic construction, aircraft fuel pumps conventionally are of a centrifugal-design employing a motor and an impeller which are enclosed within a housing. The motor is operably connected to the impeller via a drive shaft or the like, with the impeller, in turn, being coupled in fluid communication with inlet and outlet ports of the pump. During operation, the motor rotatably drives the impeller which develops a pressure drop drawing fuel or other working fluid from the associated tank through the pump inlet port and discharging the fuel, now under pressure, through the pump outlet port.
In a common construction, the impeller is provided as having an axially-extending hub or stem which is coupled to the drive shaft of the motor. Radially-extending, helical vanes are formed integrally with the hub and are enclosed by an axially-extending, generally cylindrical sleeve. The rotation of the impeller vanes within the sleeve draws the fuel or other liquid fluid into a volute chamber formed within the housing. The volute chamber converts the kinetic energy imparted to the fuel by the impeller into pressure for the discharge of the fluid through the pump outlet. Centrifugal pumps are available from a wide variety of manufacturers, including the Airborne Division of Parker-Hannifin Corp., Elyria, Ohio. Representative centrifugal pumps also are shown in commonly-assigned Chu, U.S. Pat. No. 5,427,501; Scholz, U.S. Pat. No. 5,015,156; and Lu, U.S. Pat. No. 4,813,445, as well as in Timperi et al., U.S. Pat. No. 4,877,368; Shapiro et al., U.S. Pat. No. 4,426,190; Kalashnikov, U.S. Pat. No. 4,275,988; Davis et al., U.S. Pat. No. 4,142,839; Carter, U.S. Pat. No. 3,652,186; Grennan, U.S. Pat. No. 3,806,278; Bell, U.S. Pat. No. 3,038,410; and Ridland, U.S. Pat. No. 2,846,952.
As aforementioned, certain centrifugal pumps used within aircraft fuel systems are mounted within the tank and therefore are termed in-tank or xe2x80x9cwetxe2x80x9d pumps. These pumps typically are oriented vertically within the tank, with the pump motor being located above the impeller in the direction of fuel flow. A certain minimum floor clearance generally is maintained between the impeller vanes and the bottom wall or floor of the tank to provide efficient pumping of fluid. Exemplary xe2x80x9cwetxe2x80x9d pumps are shown in U.S. Pat. Nos. 5,427,501; 5,015,156; and 2,846,952.
Alternatively, and as also was aforementioned, certain other centrifugal pumps used within aircraft fuel systems are mounted externally of the tank and therefore are termed xe2x80x9cdryxe2x80x9d pumps. These pumps, in contrast to wet pumps, may be oriented horizontally relative to the tank floor and mounted externally to the outside of the tank or to an adjacent support. A generally downwardly depending inlet tube, snorkel, hose or the like may be provided to extend in fluid communication from the pump impeller to a remote inlet port opening disposed above the tank floor. An exemplary xe2x80x9cdryxe2x80x9d pump is shown in U.S. Pat. No. 4,142,839.
With fuel pumps of either variety, spark generation and propagation into the fuel tank are major safety concerns. In this regard, it is known that during dry operation of the pump, such as with an empty fuel tank, it is possible to generate a spark caused by a dragging impeller or by debris trapped between the impeller and its surrounding sleeve. Although not known ever to have occurred, there exists at least the potential for the spark to propagate from the pump inlet into the fuel tank wherein the possibility for explosive combustion of residual fuel vapor exists. Proposed fuel pump constructions purporting to minimize spark generation and flame propagation are shown in Suzuki et al., U.S. Pat. No. 4,682,936 and Brown, U.S. Pat. No. Re. 35,404. Other techniques for improving the flame resistance of aircraft fuel systems and of combustion or turbine engines, or pumps in general are described in U.S. Pat. Nos. 5,709,187; 5,375,565; 5,357,913; 5,203,296; 4,671,060; 4,645,600; 4,676,463; 4,268,289; 3,947,362; 3,889,649; 3,911,949; 3,954,092; 3,841,520; 3,896,964; 3,635,599; and 3,434,336.
Proposals have been made for the use of flame arrestors for aircraft applications. In basic design, such arrestors are constructed as having a flame arresting element formed of a stainless steel or titanium material having a hexagonal honeycomb or a rectangular cell structure. The element, typically mounted in a housing, is installed within a fuel vent line, tank, or pump inlet to act as a barrier preventing a moving flame front from propagating into a location such as a fuel cell which may contain an explosive air/fuel mixture, while allowing for the flow of fuel or air to occur with minimal pressure drop. In having a surface area and material mass, the arrestor element functions to effect the transfer of heat from the flame front such that the temperature of the flammable mixture falls below its ignition temperature. In this way, the propagation of the flame is arrested. Commercial flame arrestors for aircraft applications are marketed by Shaw Aero Devices, Inc., Fort Myers, Fla.
Recently, concerns have been expressed over the possibility that a spark generated at a fuel pump inlet by a dragging impeller or otherwise could be propagated into the fuel tank. Indeed, it has been speculated by Tischler in Aerospace America (March, 1998), and by Taylor in the Seattle Times News (Aug. 8, 1998) that an in-tank fuel pump could have played a role in the TWA Fight 800 disaster of 1996. In response, Boeing has issued a Service Bulletin, No. 7474-28A2210 (May 14, 1998), which provides instructions in the installation of a flame arrestor at the open end of the inlet tube of the scavenge pump for the center wing tank. The United States Federal Aviation Administration also has proposed adding new airworthiness directives to 14 C.F.R. Part 39 which would make the installation of such a flame arrestor a requirement. It therefore is to be expected that a flame arrestor design which addresses such a possibility in a cost-effective manner would be well-received by the aviation industries.
The present invention is directed to a flame arresting arrangement particularly adapted for use within the fuel system of an aircraft. Within such fuel systems, a centrifugal pump is conventionally provided as including a downstream impeller and an upstream inlet port coupled in fluid communication with the impeller as received within an associated fuel tank. In accordance with the present invention, a flame arrestor body is received with the inlet port within the fuel tank. Such body is formed of an open cell, i.e., reticulated, foam material which may be a polyether- or polyester-based polyurethane elastomer. The foam has an average pore size and thickness selected as being both fluid permeable and adapted to prevent flame from propagating therethrough. With the inlet port being contained within the arrestor body, fuel within the tank may be drawn into the inlet port through such body, with any flame or spark generated by the pump impeller or otherwise being prevented from passing from the inlet port into the fuel tank. In this way, the potential of a fuel vapor ignition is reduced. Advantageously, the reticulated foam body functions both as a flame arresting device and as a fuel filter for the pump.
In a preferred embodiment, the body is configured as a shroud for the inlet port. Specifically, a downstream, first end portion of the body is configured to define an end wall portion, with a terminal end face of an opposing, upstream end portion being abuttingly supported on an interior surface of the tank and being configured as having a generally annular cross-sectional geometry. The body further extends along a central axis of the inlet port from the end face to the end wall as forming a circumferential side wall portion. The inlet port thereby is enclosed within an internal plenum defined by the end and side wall portions of said body and the interior surface of said tank. Such a construction presents a greater surface area of the body to the respective flame and fluid fronts without a corresponding increase in foam thickness. Accordingly, the potential for fouling from particulates entrained in the fuel or from xe2x80x9cicingxe2x80x9d caused by the condensation of supercooled fuel on the body is reduced without an appreciable increase in the pressure drop or other flow restriction though the body.
Such a construction, moreover, is adaptable to accommodate fuel pumps of either a xe2x80x9cwetxe2x80x9d or xe2x80x9cdryxe2x80x9d type. In this regard, the body end wall may be configured as having an aperture adapted to receive either the inlet tube of a dry, externally-mounted fuel pump, or the housing of a wet, internally-mounted pump. In a particularly preferred embodiment, the body is supported within a metal mesh outer layer which also functions to distribute fuel across the outer surface of the body.
It is, therefore, a feature of a disclosed embodiment of the present invention to provide a flame arresting system for a fuel pump of an aircraft or the like. Such pump conventionally is of a centrifugal variety which includes a downstream impeller and an upstream inlet port having a central axis, the inlet port being coupled in fluid communication with the impeller and being received within an associated fuel tank as disposed adjacent an interior surface thereof. The arresting system includes an arrestor body received with the inlet port within the fuel tank. Such body is formed of an open-cell foam material having an average pore size and thickness selected as being both fluid permeable and adapted to prevent flame from propagating therethrough. The pump inlet port is contained within the body such that fuel within the tank may be drawn into the inlet port through the arrestor body, with flame being prevented from passing into the fuel tank from the inlet port.
The present invention, accordingly, comprises the system possessing the construction, combination of elements, and arrangement of parts which are exemplified in the detailed disclosure to follow. Advantages of the invention includes a flame arresting system which is particularly adapted for aircraft applications, which may accommodate existing fuel pumps of either a wet or dry type. These and other advantages will be readily apparent to those skilled in the art based upon the disclosure contained herein.